What is Form CT1 (Corporation Tax Return)?
Form CT1 is the Corporation Tax return Irish companies file each year via ROS. It calculates the tax liability for the accounting period and is due within 9 months of the period end (specifically the 23rd of the ninth month for ROS filers). Preliminary tax of 90% (or 100% for small companies) of the final liability is due one month earlier.
Example
A company with year-end 31 December 2024 must pay preliminary tax by 23 November 2025 and file the CT1 by 23 September 2025… correction: CT1 due 23 September 2025 (9 months after year-end). Preliminary tax due 23 November 2024 (penultimate month before year-end).
Related terms
Corporation Tax is the tax Irish-resident companies pay on their profits. Trading income is taxed at 12.5%, while non-trading (passive) income is taxed at 25%. Large multinationals within the OECD Pillar Two scope pay a minimum effective rate of 15%.
Preliminary tax is Ireland's pay-on-account system. Companies pay 90% of expected current year liability (or 100% of prior year for 'small' companies with prior CT under €200,000) one month before year-end. Self-employed individuals pay 90% of current year or 100% of prior year by 31 October each year.
ROS is the Irish Revenue's online portal for tax filing and payment. All businesses must use ROS to file CT1, VAT, PAYE, RCT and other returns. Individuals use myAccount for personal taxes. Registration requires a digital ROS Cert installed in your browser.
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